Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Each pixel incorporates several, differently-colored subpixels, typically red, green, and blue, to represent each image element. A variety of flat-panel display technologies are known, for example plasma displays, field emissive displays (FEDs), liquid crystal displays (LCDs), and electroluminescent (EL) displays, such as light-emitting diode displays. To present images on these displays, the display typically receives an image input signal containing three-color-components for driving each pixel.
In emissive displays, including plasma, field-emissive and electroluminescent displays, the amount of radiant energy produced by the display is positively correlated with the amount of power that the display consumes, i.e. higher power corresponds to more radiant energy. This same relationship does not exist in transmissive displays, such as LCDs in which the light source is not modulated, as these displays typically create enough light to provide the brightest possible image and then modulate this light so that only the necessary portion of the light is transmitted to the user. However, it is known to produce LCD displays having color channel dependent light emission in which the light emission can be varied for various color channels within various regions. For example, it is known to produce LCD displays employing arrays of addressable, discrete inorganic light-emitting diodes (LEDs) as backlights and to modulate the light emission of these LEDs to affect the power consumption of the display. Within this disclosure, displays having color channel dependent light emission include emissive displays, as well as transmissive displays equipped with light sources in which light emission can be varied independently for different color channels.
These displays having color channel dependent light-emission can be produced by arranging different light-emissive materials that emit different colors of light. However, patterning these materials for some technologies, particularly small-molecule organic EL materials, is difficult for large substrates, thereby increasing manufacturing costs. One approach to overcoming material deposition problems on large substrates is to employ a single emissive material set to form, for example, a white-light emitter, together with one or more color filters in each subpixel for forming a full-color display. Such a display is taught in U.S. Pat. No. 6,987,355 entitled, “Stacked OLED Display Having Improved Efficiency” by Cok. Because the white-light emitter is modulated independently for each subpixel, this display configuration has color channel dependent light emission.
Most commonly available emissive displays employ three colors of subpixels, but it is also known to employ more than three colors of subpixels. For example, a white-light-emitting element can be included an EL display that does not include a color filter for providing a fourth subpixel, for example, as taught in U.S. Pat. No. 6,919,681 entitled, “Color OLED Display with Improved Power Efficiency” by Cok et al. U.S. 2004/0113875 A1 entitled “Color OLED display with improved power efficiency” by Miller et al. teaches an EL display design employing an unpatterned white emitter with red, green, and blue color filters to form red, green, and blue subpixels, and an unfiltered white subpixel to improve the efficiency of the device. Similar techniques have also been discussed for other display technologies.
However, since most display systems provide an image input signal having red, green, and blue color components, it is typically necessary to employ a conversion method to convert an incoming image input signal from three-color-components to a larger number of components for driving displays having four or more colors of EL subpixels. For example, Miller et al., in U.S. Pat. No. 7,230,594 entitled “Color OLED Display With Improved Power Efficiency” describe an OLED display having four light-emitting elements; including red, green, blue and white light-emitting elements together with a discussion of one such method for performing conversion of the image input signal. Miller et al. teach that when the fourth light-emitting element in an emissive OLED display has a higher power efficiency than the red, green, or blue light-emitting elements, light can be created more efficiently when it is produced by the fourth light-emitting element instead of a combination of the three red, green, and blue light-emitting elements. As such, it is possible to control the power consumption of the display by controlling the proportion of light that is produced by the red, green, and blue light-emitting elements as opposed to the white subpixel.
Miller et al. in U.S. Pat. No. 7,397,485 entitled, “Color OLED Display Having Improved Performance” further describes an emissive OLED display in which power consumption of the display can further be reduced by reducing the saturation of the displayed image under certain conditions indicated by a control signal and then using a white subpixel to provide an additional proportion of the display luminance to further reduce the power consumption of the display.
Power reduction in emissive displays can also be achieved by reducing the luminance level of the display. For example, Reinhardt in U.S. Pat. No. 5,598,565, entitled “Method And Apparatus For Screen Power Saving” discusses reducing the power to a subset of the light-emitting pixels on the display to reduce the power consumption of the display. This patent discusses determining pixels that are not critical to the task at hand and reducing the power to these pixels, which reduces the luminance of the pixels and the visibility of this portion of the display but does so only for pixels that are deemed to be less important to the user. A method for achieving a similar result is further discussed by Ranganathan et al. in U.S. Pat. No. 6,801,811, entitled “Software-Directed, Energy-Aware Control Of Display”.
Similarly, it is known to reduce the power of emissive displays under other conditions. For example, Asmus et al. in U.S. Pat. No. 4,338,623, entitled “Video Circuit with screen-burn-in protection”, issued Jul. 6, 1982 discusses a CRT display which includes a circuit for detecting a static image decreasing the brightness of the displayed image when the image is static for at least a predetermined time period. This method is disclosed with the purpose of reducing image stick artifacts, but decreases the power of the display under conditions when the display is not updated after a period of time.
In the methods for reducing the power of emissive displays through a method of driving, reducing the color saturation or luminance of the display reduces the image quality of the resulting images. Significantly reducing the luminance of the display reduces the display contrast reducing the ability of the user to see detailed information, such as text on the display. Reducing saturation of all color channels can reduce the image quality by producing washed out images.
There is a need to reduce the power consumption of EL displays without significantly reducing image quality. Further, it is desirable to increase the luminance of the display under certain circumstances, such as conditions of high ambient illumination conditions.